Many toxic gases (e.g. CO, NOx, H2S, and CH4, etc.) are harmful to human body. These toxic gases are colorless and odorless such that they cannot be detected of their presence by human senses of vision and smell. When the concentration of toxic gases exceeds a certain level in air, a person will develop symptoms, such as headache, dizziness, vomiting, or even shock and death. An alarm can be timely dispatched in the event of a toxic gas concentration exceeding the permissible level to a human body, if a gas analysis instrument or device is used to monitor the gas composition in an enclosed space or an environment with poor ventilation in real time, thereby preventing the occurrence of misfortune and disaster.
A gas sensor is a device for converting a detected gas concentration into an electric signal. Conventional gas sensors include: electrochemical gas sensors, and semiconductor metal oxide gas sensors, etc.
Although an electrochemical gas sensor can perform a detection of gas concentration at room temperature, the reference electrode thereof is liable to a chemical buildup which causes a drifting of the gas detection baseline and thus a need in recalibration and inconvenience in use.
A semiconductor metal oxide type gas sensor uses the variation of resistance caused by the amount of gas adsorbed on the surface of an oxide to monitor a variation of the gas concentration in the surrounding of the sensor. Such a gas sensor has the following advantages: good heat resistance and corrosion resistance, simple in production, easy in combination with microelectromechanical techniques, easy in transportation, low power consumption, and easy in application commercially, etc. The construction of such a gas sensor essentially includes a ceramic substrate, a detection material layer, a heater, and measurement electrodes, etc., wherein the detection material layer mainly consists of a polycrystalline and porous film of a metal oxide. For example, SnO2, ZnO [U.S. Pat. No. 4,358,951], Fe2O3, In2O3, and WO3, etc. are all suitable as a detection material layer for such a sensor. Major defects of such a sensor; however, include poor gas sensitivity, gas selectivity, and stability. Usually, in order to accelerate the desorption rate of a gas chemically adsorbed on the surface of an oxide detection material, a semiconductor metal oxide type sensor needs to be operating at a higher temperature, e.g. 300˜450° C., so that the response time of the sensor can be enhanced. However, the oxide is liable to undergo an irreversible change in its electrical properties after operating at a high temperature environment over a long period of time, thereby causing a drifting in the measured signal. Meanwhile, a few problems, e.g. size of the sensor, high electric power consumption, and maintaining at a constant temperature, etc., are also derived from the requirement of heating. As a result, such type of gas sensor tends to have higher production and operation costs.
ZnO is an n-type multifunctional semiconductor material due to its high chemical stability, low dielectric constant, and high luminous transmittance, and is widely used in dielectric ceramics, catalysts, and detection materials. Regarding the detection function, ZnO is one of the materials found and widely used in the earlier days. However, ZnO has the defects of high operating temperature (>150° C.) and poor selectivity.